Imperial College London team finds hidden oxygen losses in solar water splitting tests

Imperial College London researchers reported on 8 October 2026 that a method for measuring oxygen as it forms exposed a gap between electrical current and oxygen production in laboratory tests of hematite electrodes. The finding matters to solar water splitting research because current alone can make a material appear to produce more oxygen than direct measurement shows, especially at low reaction rates.

The work, published online in the Journal of the American Chemical Society on 26 September, examined hematite photoanodes rather than a complete hydrogen-production device. Its central result is a measurement of where the charge flowing through the test electrode did, and did not, correspond to molecular oxygen. The study does not demonstrate a practical gain in solar-fuel production.

How the team measured oxygen during water splitting

The team adapted an electrochemical mass-spectrometry system with a cell that could operate under illumination. That let the researchers illuminate a working photoanode, record its photocurrent and monitor oxygen production at the same time. The platform was developed at the Royce at Imperial facility in White City. The study reports an oxygen-measurement response time of about 300 milliseconds and sensitivity at picomoles per second.

Electrical current records charge moving through the system; oxygen measurement checks how much of that activity produces the intended gas. Comparing the two gave the researchers a way to assess oxygen Faradaic efficiency, the share of measured charge accounted for by oxygen generation. Without that comparison, current by itself can leave competing reactions or other losses hidden.

What changed as current increased on hematite

The researchers linked that change to a shift in the reaction’s measured kinetics as positive charge accumulated at the electrode surface. Their analysis found a crossover from first-order to approximately third-order water-oxidation behaviour as surface-hole density increased. At low current, they interpret the charge as contributing to processes that yield little molecular oxygen. Possible explanations include recombination or alternative surface oxidation reactions; the measurements do not establish every competing pathway.

The team also observed similar selectivity and kinetic behaviour when operating the electrode electrically in the dark. The authors say that result points to hematite surface chemistry, rather than illumination alone, as the source of the behaviour. It does not show that every metal oxide will behave the same way.

What the result means for solar-fuel research

Hematite is widely studied as a material for photoanodes, which use light to help drive the oxygen-producing side of water splitting. The finding gives researchers a more direct way to judge what happens at the electrode surface: a rise in current need not mean a matching rise in oxygen. That distinction is especially relevant when comparing materials or interpreting experiments at low reaction rates.

Flurin Eisner, who led the study with Daniele Benetti, said the equipment’s sensitivity allowed oxygen measurement at reaction rates where current alone would have given an incomplete picture. Ifan Stephens, another collaborator, said combining electrochemistry, optical spectroscopy during operation and mass spectrometry prompted the team to reassess its understanding of the reaction mechanism. Their comments describe an advance in measurement and interpretation, rather than a demonstrated improvement in fuel output.

The roughly 80% plateau also needs care. The study’s authors say oxygen efficiency below 100% may reflect the particular measurement conditions, so that value cannot be treated as a fixed ceiling for hematite. The low-current losses, meanwhile, identify a question for further experiments: which surface processes consume charge without producing measurable oxygen under those conditions?

The researchers plan to test the approach on other metal oxides and explore its use in reactions involving carbon dioxide and nitrogen. The paper says findings about water-oxidation rates in other oxides make wider relevance plausible, but direct oxygen measurements are still needed to establish whether the same selectivity relationship extends beyond hematite. For now, the supported conclusion is narrower: in these laboratory tests, measuring oxygen directly revealed behaviour that electrical current alone did not show.